Boring into the earth is an important activity in many industries, such as mining, oil and gas drilling, prospecting and tunnel construction. A wide variety of methods and various different types of bits are available for this purpose. Earth-boring typically requires rock to be broken in either or both of two principal modes, namely shear cutting or crushing, both of which methods are effected by means of rock-engaging hard inserts of some kind that are mounted onto a drill bit. Shear cutting involves inserts removing rock material by cutting into it and shearing off material. Crushing involves inserts repeatedly impacting on the rock and crushing it. Percussion drilling is an example of the latter and is widely used in mining and construction, to drill boreholes and blast holes, for example. In soft rock mining and pavement degradation, pick-like attack tools or picks may be used to break rock, concrete or asphalt.
Polycrystalline diamond (PCD) is a super-hard material comprising a mass of inter-grown diamond grains and interstices between the diamond grains. PCD is typically made by subjecting an aggregated mass of diamond grains to an ultra-high pressure and temperature. Material wholly or partly filling the interstices may be referred to as filler or binder material. PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. The sintering aid is commonly referred to as a solvent/catalyst material for diamond, owing to its function of dissolving diamond to some extent and catalyzing its re-precipitation. Consequently, the interstices within the sintered PCD product are wholly or partially filled with residual solvent/catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent/catalyst for the PCD. PCD is used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. For example, PCD inserts are widely used within drill bits used for boring into the earth in the oil and gas drilling industry.
Delwiche et al. (“Petroleum Division, ” v. 40, and “Drilling Technology” 1992, 1992, pp. 5160, “American Society of Mechanical Engineers”) disclose PCD inserts for oil and gas drilling, wherein a PCD layer is secured to a substrate comprising a diamond-impregnated, or “impreg”, back-up component. It is apparent from the diagrams and photographs that the diamond grains are coarse and sparsely distributed.
European Patent No. 0 235 455 discloses an insert for a rock bit comprising a pre-cemented tungsten carbide body having a cylindrical grip length extending along a major portion of the insert. The above-mentioned reference also discloses a progressive decrease in the modulus of elasticity through transition layers from an outer ultra-hard diamond layer to the carbide body, in order to reduce spalling.
U.S. Pat. No. 4,811,801 discloses a rock bit insert including a polycrystalline diamond surface on an insert body having a head portion made from a material with elasticity and thermal expansion properties advantageously tailored for use in three types of rock bits, as well as the three types of rock bits made with such inserts.
U.S. Pat. No. 5,304,342 discusses that for a given application, it is desirable to provide the stiffest possible WC—Co cemented carbide substrate, that is, one with a high modulus of elasticity, thereby minimizing the deflection of the PCD layers and reducing the likelihood of PCD failure. However, if the modulus of elasticity is too high, the inserts are prone to break off during drilling.
U.S. Pat. No. 6,258,139 discloses a PDC (polycrystalline diamond compact) with an internal diamond core in the substrate, to provide additional diamond for exposure when the substrate is sufficiently eroded. Also disclosed, is a PDC with an internal carbide core, which is entirely enclosed by the diamond region of the PDC cutter, to avoid high tensile stresses in the diamond region.
There is a need to minimize the thickness of super-hard material in attack tool inserts and consequently their costs while achieving high impact resistance. This is especially important for rock or pavement degradation or drilling applications.